1. Field of the Invention
This invention relates generally to locating electrical short-circuits and ground faults in a rotor winding of an electrical generator and, more particularly, to a method for locating inter-turn shorts or ground faults in the rotor winding which analyzes data from a Recurrent Surge Oscillograph (RSO), identifies a spike or anomaly in a reflected RSO signal, determines an elapsed time between a transmitted signal and the anomaly in the reflected signal, and calculates a location of the inter-turn short or ground fault based on the elapsed time.
2. Description of the Related Art
Electrical generators of the type used by utilities to generate electricity are large and complex machines. Although the reliability and efficiency of these machines has improved over the years, there are still some generator maintenance issues which require regular monitoring. Among the most prominent maintenance issues with electrical generators are inter-turn short circuits and ground faults in the rotor winding.
Most electrical machines include a rotor with a copper winding. In the case of a large electrical generator, the rotor winding is constructed of heavy-gauge copper bar, rectangular in cross-section, which is formed in the shape of multiple coils. Each of the several coils contains several layers or “turns” stacked on top of one another, with an insulating material between the turns. The rotor winding is subjected to a variety of loads and stresses—including centrifugal forces, thermal expansion/contraction, and vibration—which can lead to deterioration and breakdown of the insulation. When the insulation breaks down, a short circuit between adjacent turns of the winding, or inter-turn short circuit, results. In other circumstances, rotor windings can develop a short circuit to ground, or ground fault.
Various methods have been developed to diagnose inter-turn short circuits and ground faults in generator rotor windings. One of the most flexible and widely-used diagnostic tools for rotor winding faults is the Recurrent Surge Oscillograph (RSO) test. The RSO test can be used with the rotor in position in the generator frame, thereby eliminating costly disassembly of the generator. The RSO test can also be used with the rotor spinning, and can therefore detect inter-turn short circuits and ground faults which only appear when the winding is subjected to centrifugal force. In the RSO test, a high-frequency, low-voltage waveform is inserted at both ends of the winding, and the differences in the returning signals are evaluated.
An experienced technician can identify one or more inter-turn short circuits or ground faults by examining the RSO test output. The technician may even be able to estimate which coil of a winding has the inter-turn short circuit or the ground fault. However, even for an experienced RSO test technician, the waveforms produced as RSO test output do not provide sufficient resolution to accurately determine the location of the inter-turn short circuit or ground fault in the rotor winding. A method of rotor winding fault diagnosis is needed which does not require an experienced technician to evaluate the results, and which enables the location of the fault to be defined—including not only the coil number, but also the turn within the coil and the position within the turn.